This invention was made in the course of, or under, a contract with the United States Energy Research and Development Administration. The invention relates generally to gas-absorption processes -- i.e., processes in which a gas mixture is contacted with a liquid absorbent (solvent) for preferentially dissolving one or more components of the mixture. More particularly, the invention relates to continuous gas-absorption processes, in which a liquid absorbent is circulated continuously through a closed-loop system which includes gas-absorption, gas desorption, and absorbent-regeneration zones.
This invention is applicable, for example, to separations of the kind described or referenced in coassigned U.S. Pat. No. 3,762,133, issued on Oct. 2, 1973. Briefly, that patent discloses the use of liquid fluorocarbons as preferential absorbents (solvents) for selected components of various feed-gas mixtures, the selected components including xenon, krypton, oxygen, iodine, methyl iodide, and the lower oxides of carbon, nitrogen, and sulfur. The typical separation is conducted in a closed-loop system including a plurality of serially connected columns through which the liquid absorbent is circulated continuously. The system includes an absorption column, a fractionator column, and a stripper column, the last two columns being provided at their upper ends with condenser-containing systems for venting desorbed gases and at their lower ends with solvent reboilers. The liquid absorbent outflow from the absorber column usually is heated before introduction to the fractionator column. Typically, the three columns are operated at appreciably different superatmospheric pressures, and thus pressure-reducing valves are required in the liquid-flow lines between the columns. The pressure in the stripper column is much lower than that in the absorber column, so that a high-pressure-ratio pump is required to recycle the liquid solvent from the stripper to the absorber. The feed-gas mixture is introduced to the absorber, where the selected component preferentially dissolves in the liquid absorbent and is retained in the absorbent as it is passed through the fractionator and into the stripper. Depending on its solubility relative to other gases dissolved in the absorbent, the selected component either (a) desorbs in the stripper and is withdrawn therefrom as a gas or (b) remains in the liquid-absorbent outflow from the stripper and subsequently is recovered therefrom, as by distillation.
Gas absorption processes of the kind just described require the use of process systems which are not as compact, simple, or reliable as desired. This is due mainly to the use of a large number of system components and control systems therefor. As a specific example, the fractionator-column requires (among other things): a pressure-reducing valve in the line conveying liquid absorbent from the absorber to the fractionator; a system for heating the flow in that line to a selected temperature; a flash chamber in that line; a differential-pressure indicator for the liquid in the column; a pressure-reducing valve in the off-gas line to control column pressure; a condenser-containing system for venting desorbed gases; a refrigeration system for the condenser; a solvent reboiler; and reboiler-heater and level-control systems. Another disadvantage of the conventional system is that a high-pressure-ratio pump is required for recycling the liquid solvent from the stripper to the absorber. Another disadvantage is that the pressure-reducing valve between the absorber and fractionator tends to plug with ice if there is appreciable water vapor in the feed gas to the absorber. The disadvantages of the conventional gas-absorption systems are of special concern in applications where the feed gas contains radioactive components, because in that event process reliability is of paramount importance and the entire system must be housed in a "hot cell," where space is at a premium and in which repair or replacement of system components is difficult.